The present invention relates to an electric power steering apparatus using a motor as a source for generating steering assisting force.
An automobile is steered by transmitting an operation for rotating a steering wheel provided in a vehicle compartment to a steering mechanism provided outside the vehicle compartment for turning a wheel (generally a front wheel) for steering. FIG. 1 is a sectional view showing a conventional electric power steering apparatus and FIG. 2 is a sectional view showing a speed reducing mechanism portion. For example, as shown in FIG. 1, an electric power steering apparatus for an automobile comprises a first steering shaft 101 having an upper end coupled to a steering wheel 100 for steering, a second steering shaft 103 having an upper end coupled coaxially to a lower end of the first steering shaft 101 through a torsion bar 102 and a lower end coupled to a steering mechanism connected to a wheel, a torque sensor 104 for detecting a torque applied to the first steering shaft 101 by the rotation of the steering wheel 100 through a twist generated on the torsion bar 102, a steering assistance motor 105 to be driven based on the result of detection of the torque sensor 104, and a speed reducing mechanism having a worm 106 and a worm wheel 107 which are connected to an output shaft of the motor 105 and serve to reduce the speed of the rotation of the output shaft and to transmit the rotation to the second steering shaft 103, and the operation of the steering mechanism corresponding to the rotation of the steering wheel 100 is assisted by the rotation of the motor 105, thereby relieving a driver""s labor and burden for steering.
The worm 106 constituting the speed reducing mechanism is supported in a fit hole of a housing 110 through a pair of rolling bearings 108 and 108 as shown in FIG. 2, and the second steering shaft 103 provided with the worm wheel 107 is supported in the fit hole of the housing 110 through a pair of rolling bearings 109 and 109 so that the movement of the worm 106 and the worm wheel 107 in radial and axial directions is blocked.
In the steering apparatus using the worm 106 and the worm wheel 107, thus, when the backlash amount of an engagement portion is large, a backlash sound is generated so that the backlash sound leaks into the compartment of the automobile. In order to reduce the backlash amount of the engagement portion of the worm 106 and the worm wheel 107 and to eliminate a beating sound through the backlash, the worm 106, the worm wheel 107, the rolling bearings 108 and 109, the second steering shaft 103 and the housing 110 which are processed such that a distance between the rotation centers of the worm 106 and the worm wheel 107 is coincident with a distance between the centers of the fit holes in which the rolling bearings 108 and 109 are fit within an allowance are selected and assembled. A long time is required for the assembly. Furthermore, while a high load is applied to rotate the worm 106 and the worm wheel 107 and a loading work for adapting the engagement portion after the assembly, a long time is required for the loading work so that an improvement has been demanded.
Moreover, the wear of the teeth of the worm 106 and the worm wheel 107 is increased, the atmospheric temperature of the worm wheel 107 formed of a synthetic resin is changed or the worm wheel 107 absorbs moisture so that the backlash amount is increased. Furthermore, there has been a problem in that the torque of the engagement portion is disordered and stock is increased through the selective assembly described above.
The present applicant has proposed an electric power steering apparatus (Japanese Patent Application Laid-Open No. 2000-43739) in which moving means (pressing means) such as an elastic body for moving, in a radial direction, a shaft on the opposite side to a motor in a worm having a motor side shaft rotatably supported in a housing through a rolling bearing, moving the bearing in abutment on the peripheral surface of a bearing fitted in the shaft and pressing the worm toward a worm wheel is provided and a distance between the rotation centers is regulated by operating the pressing means so that a backlash amount can be adjusted.
Moreover, the Japanese Patent Application Laid-Open No. 2000-43739 has also described an electric power steering apparatus in which pressing means for moving, in a radial direction, a motor side shaft of a worm having a shaft on the opposite side to the motor rotatably supported on a housing through a rolling bearing, moving the bearing in abutment on the peripheral surface of a bearing fitted in the shaft and pressing the worm toward a worm wheel is provided, a coupling portion of the motor side shaft of the worm and the output shaft of the motor is coupled through a constant velocity joint so that the worm can be slid around a bearing portion having the shaft on the opposite side to the motor supported thereon, the distance between the rotation centers is regulated by operating the pressing means so that a backlash amount can be adjusted.
On the other hand, as described in Japanese Utility Model Publication No. 2-8124 (1990), there has been known a worm type speed reducing machine in which pressing means having an insertion hole for inserting a bearing fitted in a shaft provided on one of ends of a worm and capable of moving in the radial direction of the shaft is provided such that the worm is pressed toward a worm wheel through the pressing means or pressing means having a pair of rollers abutting on two peripheral positions of the worm and capable of moving in the radial direction of the worm such the that worm is pressed toward the worm wheel through the pressing means.
However, the conventional pressing means is formed to be smoothly moved in a guide hole provided in a housing accommodating a worm, while the tooth trace of the worm wheel 107 of the speed reducing mechanism or the like is twisted in a rotation direction with respect to a rotation center line. Accordingly, when a rotation torque is to be applied from the worm 106 to the worm wheel 107, in other words, the steering is to be assisted through the rotation of the motor 105, a comparatively great component of force is generated such that the worm 106 is moved in a radial direction along the tooth trace of the worm wheel 107 so that the worm 106 is strongly pressed in the radial direction through the component of force. For this reason, in the conventional electric power steering apparatus having such a structure that the backlash amount is reduced by using the pressing means as described above, there has been a problem in that the pressing means is moved in a direction crossing the moving direction in the guide hole by the pressing force (component of force) applied to the worm 106 and abuts on the guide hole to make a sound.
In the electric power steering apparatus disclosed in the Japanese Patent Application Laid-Open No. 2000-43739, moreover, the pressing means provided in the guide hole is fitted in the shaft on one of the ends of the worm and simply abuts on the peripheral surface of the bearing movable in the radial direction. The pressing means does not block the movement of the bearing in a direction crossing the direction of movement of the pressing means. Therefore, when the pressing means presses the worm toward the worm wheel and the steering is assisted in the state in which the rotation center line of the worm is inclined with respect to the rotation center line during non-press, the worm is moved in the radial direction with respect to the rotation center line during the press by the component of force in the radial direction described above so that the worm is eccentrically rotated, in other words, carries out whirling. Consequently, there is also a problem in that the defective engagement of the engagement portion is caused and torque nonuniformity is generated. Moreover, it is necessary to provide pressing means for abutting on the bearing fitted in the shaft of the worm and the circumferential surface of the bearing. Consequently, there is also a problem in that the number of parts is increased, resulting in an increase in a cost.
In the worm type speed reducing machine disclosed in the Japanese Utility Model Publication No. 2-8124, moreover, the shaft on one of the ends of the worm is fitted and supported on the other end of the pressing means having one of the ends provided in the guide hole and the other end extended toward the outside from the guide hole, in other words, the shaft potion is overhung for the guide hole or the other end extended toward the outside from the guide hole supports the worm through a pair of rollers. The shaft or the worm does not block the movement in the direction crossing the direction of movement of the pressing means. Therefore, when the pressing means presses the worm toward the worm wheel and the steering is assisted in the state in which the rotation center line of the worm is inclined with respect to the rotation center line during the non-press, the worm is moved in the radial direction with respect to the rotation center line during the press by the component of force in the radiation direction described above and is eccentrically rotated, in other words, carries out whirling. Consequently, there is also a problem in that the defective engagement of the engagement portion is caused and torque nonuniformity is generated. Moreover, it is necessary to provide a pair of rollers on the pressing means. Consequently, there is a problem in that the number of parts is increased, resulting in an increase in a cost.
In the electric power steering apparatus disclosed in the Japanese Patent Application Laid-Open No. 2000-43739, moreover, in the case in which a steering wheel gets on a curb or the like so that a reverse input load is transmitted from the steering wheel through the steering shaft, the worm is pressed to go away from the worm wheel in the radial direction. In the electric power steering apparatus in which the backlash amount is reduced by the pressing means as describe above, accordingly, there is a possibility that the reverse input load might be applied from the worm to the pressing means so that the pressing means is excessively deformed elastically, and furthermore, the center of rotation of the worm might be inclined excessively so that shoulder getting is generated on the rolling bearing supporting the other end of the worm or the worm is broken.
In the electric power steering apparatus disclosed in the Japanese Patent Application Laid-Open No. 2000-43739, moreover, the shaft on one end side of the worm can be moved in the radial direction and the shaft on the other end side is simply supported rotatably by using a general rolling bearing. Therefore, the regulation margin of the distance between the rotation centers through the pressing means depends on the corner clearance of the general rolling bearing, in other words, the inclination of the inner ring of the rolling bearing. Accordingly, the regulation margin of the distance between the rotation centers, and furthermore, the regulation margin of the backlash amount are restricted. In the electric power steering apparatus, moreover, in the case in which the steering wheel gets on a curb or the like so that the reverse input load is transmitted to the worm from the steering wheel through the steering shaft, the pressing means side of the worm is moved to go away from the worm wheel in the radial direction and carries out whirling. At this time, in the electric power steering apparatus in which the regulation margin of the distance between the rotation centers depends on the corner clearance of the bearing as described above, in the case in which the whirling amount of the worm is comparatively increased through the reverse input load, there is a possibility that the shoulder getting might be generated on the rolling bearing supporting the other end of the worm, resulting in a breakage of the rolling bearing.
It is an object of the present invention to solve the above-mentioned problems and to provide an electric power steering apparatus capable of eliminating a sound at a low cost.
It is another object of the present invention to solve the above-mentioned problems and to provide an electric power steering apparatus capable of preventing pressing means, a bearing, a worm and the like from being broken due to a reverse input load.
A first aspect of the present invention is directed to an electric power steering apparatus, comprising a driving gear interlocking with rotation of a motor for steering assistance and having a shaft movable in a radial direction thereof, a driven gear engaged with the driving gear and connected to a steering mechanism, and a pressing body pressing the driving gear toward the driven gear by moving the shaft in the radial direction, wherein the pressing body has a contact portion to abut on a peripheral surface of the shaft and an elastic body for restricting movement of the pressing body in a direction crossing the radial direction is provided around the pressing body. When the driving gear is pressed in the radial direction along the tooth trace of the driven gear and the pressing force is applied to the pressing body, the movement of the pressing body can be restricted by the elastic body provided around the pressing body. Therefore, it is possible to eliminate a sound made by the movement of the pressing body to hit the housing. In addition, the pressing body abuts on the shaft of the driving gear. Therefore, it is possible to reduce the number of parts as compared with the conventional arts using a pressing body or a roller to abut on the bearing. Consequently, a cost can be reduced.
In the electric power steering apparatus according to the first aspect, the contact portion is provided in an approximate middle position in the radial direction of the pressing body and the elastic body is provided in both side positions in the radial direction with being separated from the contact portion and/or the shaft. When the driving gear is pressed in the radial direction along the tooth trace of the driven gear and the pressing force is applied to the pressing body, the movement of the pressing body can be restricted by the elastic body. Therefore, the pressing body can be prevented from being inclined and can be maintained in such a state as to be moved in the radial direction.
A second aspect of the present invention is directed to an electric power steering apparatus, comprising a driving gear interlocking with rotation of a motor for steering assistance and having a shaft movable in a radial direction thereof, a driven gear engaged with the driving gear and connected to a steering mechanism, and a pressing body pressing the driving gear toward the driven gear by moving the shaft in the radial direction, wherein the pressing body has a contact portion to abut in at least two circumferential positions of the shaft. The contact portion abuts in the two circumferential positions of the shaft to press the driving gear toward the driven gear. Therefore, when the driving gear is pressed in the radial direction along the tooth trace of the driven gear, it is possible to prevent the shaft from being moved with respect to the pressing body and to eliminate a sound made by the hit of the shaft on the pressing body. In addition, the pressing body abuts on the shaft of the driving gear. Therefore, it is possible to reduce the number of parts as compared with the conventional arts using a pressing body or a roller to abut on the bearing. Consequently, a cost can be reduced.
In the electric power steering apparatus according to the second aspect, the contact portion is provided in an approximate middle position in the radial direction of the pressing body and an elastic body for restricting movement in a direction crossing the radial direction of the pressing body is provided in both side positions in the radial direction with being separated from the contact portion and/or the shaft. When the driving gear is pressed in the radial direction along the tooth trace of the driven gear and the pressing force is applied to the pressing body, the movement of the pressing body can be restricted by the elastic body. Therefore, the pressing body can be prevented from being inclined and can be maintained in such a state as to be moved in the radial direction.
A third aspect of the present invention is directed to an electric power steering apparatus, comprising a driving gear interlocking with rotation of a motor for steering assistance and having a shaft movable in a radial direction thereof, a driven gear engaged with the driving gear and connected to a steering mechanism, a pressing body pressing the driving gear toward the driven gear by moving,the shaft in the radial direction, and a guide member provided with a guide hole for guiding movement of the pressing body, wherein the pressing body has a fit hole having the shaft fitted therein in an approximate middle position in the radial direction along the guide hole and has a motion block portion for blocking movement in a direction crossing the radial direction in contact with the guide hole in both end portions in the radial direction with respect to the fit hole. The motion block portion provided in both end portions in the radial direction along the guide hole of the pressing body with respect to the fit hole in which the shaft of the driving gear is fitted comes in contact with the guide hole to block the movement of the pressing body in a, direction crossing the radial direction, thereby supporting the shaft with respect to the guide hole. Therefore, when the pressing body presses the worm toward the worm wheel and the steering assistance is carried out with the rotation center line of the worm inclined with respect to the rotation center line during non-press, it is possible to prevent the worm from being moved in the radial direction with respect to the rotation center line during press through a component of force in the radial direction. Accordingly, the whirling operation of the worm can be prevented and the engagement portion can be engaged well, and a torque nonuniformity can be eliminated. Moreover, the pressing body supports the shaft. Therefore, it is possible to reduce the number of parts as compared with the conventional art in which the shaft is supported by using a bearing. Thus, a cost can be reduced.
In the electric power steering apparatus according to the third aspect, a plain bearing for permitting a slippage in an axial direction of the shaft is provided in the fit hole portion. The shaft of the worm is fitted in the plain bearing. Therefore, even if the component of force of the pressing force in the radial direction which is to be applied to the worm is applied in the axial direction, the worm can be moved smoothly in the axial direction and the rotation resistance of the worm can be reduced.
In the electric power steering apparatus according to the third aspect, a clearance for permitting an inclination of the shaft with respect to a center of the fit hole is provided between the fit hole and the shaft. When the pressing body presses the worm toward the worm wheel and the steering aiding is carried out with the rotation center line of the worm inclined with respect to the rotation center line during the non-press, the shaft of the worm can be inclined well and the rotation resistance of the worm can be reduced.
A fourth aspect of the present invention is directed to an electric power steering apparatus, comprising a driving gear movable in a radial direction thereof interlockingly with rotation of a motor for steering assistance, a driven gear engaged with the driving gear and connected to a steering mechanism, a pressing body pressing the driving gear toward the driven gear, and restricting means for restricting separating movement of the driving gear with respect to the driven gear. The backlash amount of the engagement portion can be decreased by the pressing body. In addition, a reverse input load applied to the driving gear is applied by the restricting means provided separately from the pressing body, thereby restricting the separating movement of the driving gear with respect to the driven gear. Therefore, it is possible to prevent the pressing body from being broken due to the reverse input load and to well prevent shoulder getting from being generated on the bearing supporting the driving gear or the worm from being broken.
A fifth aspect of the present invention is directed to the electric power steering apparatus according to the fourth aspect, further comprising a supporting member for supporting the driving gear, wherein the restricting means is provided on at least one of the driving gear and the supporting member. The restricting means can be provided on the supporting member of the driving gear. Therefore, the structure of the restricting means can be simplified, the processing and assembling workability of the restricting means can be enhanced and a cost can be reduced comparatively.
A sixth aspect of the present invention is directed to the electric power steering apparatus according to the fifth aspect, wherein the driving gear includes a shaft on one of ends in an axial direction, the supporting member has a concave hole in which the shaft is to be inserted and a holding hole facing an inside of the concave hole and holding the pressing body, and the restricting means is provided on at least one of the shaft and the concave hole. The restricting means is provided on at least one of the shaft of the driving gear and the concave hole in which the shaft is to be inserted. Therefore, the restricting means can be formed to have a comparatively small size, and it is possible to avoid an increase in the size of the speed reducing mechanism portion through the restricting means.
In the electric power steering apparatus according to the sixth aspect, the restricting means is a ring attached to the inside of the concave hole. The ring is attached to the concave hole having a comparatively small diameter in which the shaft of the driving gear is to be inserted. Therefore, it is possible to simply provide the restricting means without changing the existing supporting structure of the driving gear.
In the electric power steering apparatus according to the sixth aspect, the restricting means is a thicker-diameter portion formed integrally with the shaft. When the shaft of the driving gear is to be processed, the restricting means can be provided together with the shaft. Therefore, the work for assembling the restricting means is not required. Consequently, it is possible to reduce a cost still more.
In the electric power steering apparatus according to the sixth aspect, the restricting means is a ring attached to the shaft. The ring is attached to the shaft, therefore, it is possible to simply provide the restricting means without changing the existing supporting structure of the driving gear.
A seventh aspect of the present invention is directed to an electric power steering apparatus, comprising a driving gear supported rotatably in a housing through a bearing interlockingly with rotation of a motor for steering assistance, a driven gear engaged with the driving gear and connected to a steering mechanism, a pressing body pressing the driving gear toward the driven gear at a free end side of the driving gear, and a convex provided on at least one of an inner peripheral side portion and an outer peripheral side portion in the bearing and permitting a rocking operation of the driving gear. The convex provided on at least one of the inner peripheral side portion and the outer peripheral side portion in the bearing supporting the worm in the housing permits the rocking operation of the driving gear. Therefore, in such a structure that the worm is rocked by the pressing body for moving the driving gear in the radial direction, the worm can be rocked at a rocking angle which is equal to or greater than the corner clearance of the bearing. Thus, the regulation margin of the backlash amount can be increased. Moreover, in the case in which the reverse input load is transmitted to the driving gear, the driving gear can be rocked at a rocking angle which is equal to or greater than the corner clearance of the bearing by using the convex as a fulcrum. Therefore, it is possible to prevent the bearing from being broken due to the reverse input load.
In the electric power steering apparatus according to the seventh aspect, the housing and the driving gear have a fit portion in which the bearing is to be fitted and the convex is formed such that a sectional shape in an axial direction of the fit portion is arcuate. The fit portion of the housing and the driving gear is formed to be arcuate. Therefore, a special part is not required and a cost can be reduced. In addition, an existing bearing can be used, and furthermore, the axial length of the driving gear can be shortened comparatively.